Pressure seal

ABSTRACT

An arrangement for eliminating leakage of ash and combustibles bearing flue gases from within a vapor generator gas pass enclosure to a compartment situated outside of the enclosure including the means for maintaining a higher flue gas pressure in the compartment as compared to the pressure within the gas pass enclosure.

United States Patent [191 Watanabe et al.

[ June 25, 1974 PRESSURE SEAL [75] Inventors: Tsuneo Watanabe; Akinori Iwai,

both of Hiroshima, Japan [73] Assignee: Babcock & Wilcox Limited, London, England 1 [22] Filed: June 20, 1972 [21] Appl. No.: 264,675

[52] U.S. Cl. 122/494, 122/235 G [51] Int. Cl. F22b 37/22 [58] Field of Search 122/235 G, 494, 479 A; 165/70 [56] References Cited v UNITED STATES PATENTS 2,882,871 4/1959 Koch 122/479 A 3,115,123 12/1963 Smith 122/478 3,155,078 11/1964 Clayton, .lr. 122/479 A 3,174,464 3/1965 Johnson 122/235 B 3,267,908 8/1966 Sharan 1 10/49 3,310,036 3/1967 Frendberg 110/49 R 3,395,676 8/1968 Sprague 165/70 3,670,810 6/1972 Allen, Sr 165/70 FOREIGN PATENTS OR AlPPLlCATlONS 158,957 9/1954 Australia 122/479 Primary Examiner Kenneth W. Sprague Assistant Examiner- Henry C. Yuen Attorney, Agent, or Firm-Joseph M. Maguire, Esq.;

Robert J. Edwards, Esq.

[57] ABSTRACT An arrangement for eliminating leakage of ash and combustibles bearing flue gases from within a vapor generator gas pass enclosure to a compartment situated outside of the enclosure including the means for maintaining a higher flue gas pressure in the compartment as compared to the pressure within the gas pass enclosure.

3 Claims, 2 Drawing Figures If 0 f PRESSURE SEAL BACKGROUND OF THE INVENTION This invention relates generally to vapor generating and superheating units of the type which have fluid heating tubes extending from the gas pass enclosure through the boundary thereof and terminating in a compartment on the outside of the enclosure. More particularly the invention relates to an arrangement for eliminating leakage of ash and combustibles bearing flue gases from the gas pass enclosure to the compartment.

The gas pass enclosure of a conventional vapor gen erating and superheating unit comprises a furnace chamber, convection and outlet gas passes, associated boundary walls and a horizontally extending roof. The gas pass enclosure is at least partly formed of tubular walls and includes convection heating surfaces such as a superheater, reheater and economizer which may have tube ends extending through the enclosure for connection to headers housed in compartments formed on the outside of the enclosure.

The supporting structure associated with these units includes hangers which attach to the above-mentioned headers to provide a fixed support. As the unit is brought up to operating temperature, the tubes connected to these headers undergo thermal expansion from the point of restraint at the header. There is also thermal expansion of the header itself which tends to cumulatively increase the spacing of the tubes associated therewith. Thus it has been necessary to maintain a clearance between the penetrating tubes and the edges of the penetration holes in the roof and boundary walls to permit unrestrained longitudinalmovement of these tubes. However, these required thermal expansion clearances have permitted some leakage of the combustion gases from the gas pass enclosure into the header compartments. This leakage problem has been a constant source of maintenance difficulty where deposits of ash and combustibles would tend to accumulate in the header compartments thereby hindering inspection and creating a fire hazard. 1

Prior art efforts at dealing with this problem have no been altogether satisfactory. One approach has been to introduce air into the header compartments so as to maintain the compartment pressure above that of the gases flowing through the gas pass enclosure. This arrangement carries the risk of gas leakage into the compartments whenever pressurizing air is cut off; more over, the use of air has proven uneconomical from the standpoint of increased fan operating and capital costs and thermally inefficient as a result of continued leakage of relatively cooler air into the combustion gas stream.

SUMMARY OF THE INVENTION In accordance with the present invention, a novel arrangement is taught which eliminates substantially all of the leakage of combustion gases from the gas pass enclosure to the header compartments. This is achieved by providing a duct having its intake at the outlet gas pass and separately discharging into each An alternate embodiment is taught with respect to units equipped with a gas recirculation system, wherein the duct has its intake downstream of the gas recirculation fan discharge thereby dispensing with the requirement of a pressurizing fan.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional side elevation of a vapor generating and superheating unit embodying the invention.

FIG. 2 is a sectional side elevation of a vapor generat ing and superheating unit depicting an alternate embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, each illustrates a forced flow once-through steam generating and superheating unit for central station use. The following description is concerned with like components and is therefore applicable to both units.

The main parts of each unit include an upright chamber 1 of substantially rectangular horizontal crosssection defined by water-cooled walls 2 and a portion of water-cooled roof 4 and having a gas outlet at its upper end fonned above an arch 8 and opening into an upright gas passage 6 of substantially rectangular horizontal cross-section. The lower ends of the furnace front and rear walls slope inwardly and downwardly to form a hopper bottom 3. Fuel firing equipment (not shown), is normally disposed along the lower portion of the furnace front and rear walls at levels above the hopper 3. Combustion air is supplied through the windboxes 5. The gas passage 6 has an outlet at its lower end opening into a horizontally extending gas passage 7 of substantially rectangular vertical cross-section and leading to a discharge stack (not shown).

An upper compartment or penthouse 9 extends upwardly from the roof -4 to form an insulated chamber for housing headers, tubes and piping associated with the unit. Additional compartments as shown at 17 house headers at an intermediate level along the furnace walls.

The fluid flow circuitry includes a mix header 16 disposed within the compartment 17 at an intermediate level along the furnace front wall and flow connected to tubes associated therewith; a secondary superheater 10 located above the arch 8 and adjacent to the furnace chamber outlet and having inlet and outlet headers 11 disposed within the penthouse 9; a reheater 12 located in the gas passage 6 and having inlet and outlet headers 13 disposed within the penthouse 9; and primary superheater 14 and economizer 15 also located in the gas passage 6 and having respective inlet and outlet headers (not shown) located in the penthouse 9. The tubes, flow connecting the aforementioned heating surfaces to their respective inlet and outlet headers, extend through appropriate openings in the water-cooled roof The vapor generating and superheating unit shown in FIG. 1 includes a gas recirculation system for the purpose of steam temperature control. The gas recirculation system comprises an intake duct 21 communicating at its inlet end with the gas passage 7, preferably at the top side thereof, and having its outlet end communicating with the suction side of a gas recirculation fan 23. The intake duct 21 is equipped with a mechanical dust collector 22 of the kind well known in the art, e.g.,

a multi-cyclone or baffle type collector. The gas recirculation fan 23 pressurizes that portion of combustion or flue gases withdrawn from the gas passage 7 and delivers the pressurized gas to the furnace chamber 1 by way of hopper 3 and a discharge duct 24 communicating therewith. The quantity of recirculated gas is controlled by dampers (not shown) which are operatively associated with the fan 23.

In accordance with the invention, there is shown a conduit 25 having its inlet end communicating with the discharge duct 24, preferably at the top side thereof, and having its outlet end divided into a branch conduit 25a leading to the penthouse 9 and a branch conduit 25b leading to the compartment 17.

The quantity of pressurized gas which is admitted to the penthouse 9 and the compartment 17 is regulated through the respective damper devices 26a and 26b associated with the branch conduits 25a and 25b. Each of the damper devices 26a and 26b are automatically controlled in accordance with the magnitude of the differential pressure between the gas pass enclosure and the compartment associated therewith as compared to a predetermined pressure value. Thus, whenever the differential gas pressure between the penthouse 9 as measured at a point 27a and the furnace chamber 1 as measured at a point 27a exceeds the predetermined pressure value, the damper 26a will modulate toward a closed position thereby reducing and eventually shutting off the flow of combustion gases to the penthouse 9. Conversely, whenever the magnitude of the differential gas pressure is less than the predetermined pressure value, the damper 26a will modulate toward a full open position to allow the flow of combustion gases to the penthouse 9. Similarly, the flow of combustion gases to the compartment 17 is regulated by the damper 26b which is automatically controlled in accordance with the magnitude of the differential pressure between the furnace chamber 1 as measured at point 27b and the compartment 17 as measured at point 2712 when compared to the predetermined pressure value.

Safety valves 28a and 2812 are included in the branch conduits 25a and 2512, respectively, to provide a means for releasing excessive pressure in the penthouse 9 and compartment 17 which may result from the gas being heated during cold start-up of the vapor generator.

Referring to FIG. 2, there is shown a vapor generating and superheating unit equipped with a rotary regenerative type air heater 41 which is arranged to absorb heat from the gases flowing through the passage 7 and transfer this heat to the combustion air being conveyed by a duct 42 to the windboxes 5. In accordance with an alternate embodiment of the invention, an intake conduit 43 communicates with the gas passage 7, preferably at the top side thereof, downstream gas flow-wise of the air heater 41. The conduit 43 includes a mechanical dust collector 44 of the type referred to in FIG. 1 and a pressurizing fan 45. A discharge conduit 46 communicates with the outlet of fan 45 and leads into branch conduits 46a and 46b which in turn communicate with the penthouse 9 and compartment 17, respectively. The quantity of pressurized gas which is admitted to the penthouse 9 and the compartment 17 is regulated through respective damper devices 26a and 26b associated with the branch conduits 46a and 461;. Each of the damper devices 26a and'26b are automatically controlled in accordance with the magnitude of the differential pressure between the gas pass enclosure and the compartment associated therewith as compared to a predetermined pressure value. The regulation of flow of combustion gas to the penthouse 9 and the compartment 17 is in the same manner as described with reference to FIG. 1.

During the operation of the vapor generating and super-heating unit, the present arrangement permits pressurization of the penthouse 9 and compartment 17 with relatively clean combustion gases as compared to those in the furnace chamber 1. The pressurization thus achieved insures that, if any gas leakage should occur between the penthouse or compartment and the gas pass enclosure, it will always be in the direction away from the penthouse or compartment and into the gas pass enclosure thereby substantially eliminating combustibles and ash deposition in the penthouse. The pressurized gas is withdrawn from a region of relatively clean gas, e.g., the top side of a duct and is passed through a mechanical dust collector before being conveyed to the penthouse or compartment so that the quantity of entrained combustibles and ash conveyed to the penthouse or compartment is negligible.

While in accordance with provisions of the status there is illustrated and described herein a specific embodiment of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

What is claimed is:

1. In combination with a vapor generating and superheating unit, structure defining a gas space including a furnace chamber and a passageway situated downstream gas flow-wise of the furnace, means for passing a combustion gas through the gas space, a plurality of fluid heating tubes exposed to said combustion gas, and

wherein at least some of the tubes have end portions thereof terminating outside of said gas space, the structure including a plurality of openings, each of the openings being sized to slidably receive a respective one of said tube portions while preventing movement thereof transverse to the central axis of the opening, header means flow connected to said end portions, wall means cooperating with the structure to form at least one compartment enclosing said header means, conduit means for conveying a portion of the combustion gas to said compartment, and means for maintaining the combustion gas within said compartment at a higher pressure level than that of said gas space thereby sealing the compartment from said gas space.

2. The combination according to claim 1 wherein said last named means includes a duct situated outside of the gas space and communicating with said furnace and passageway, a fan disposed in said duct for recirculating aportion of the combustion gases from the passageway to the furnace, and said conduit means being flow connected to the duct at a point downstream gas flow-wise of said fan.

3. The combination according to claim 1 wherein said conduit means is flow connected to the passageway. I 

1. In combination with a vapor generating and superheating unit, structure defining a gas space including a furnace chamber and a passageway situated downstream gas flow-wise of the furnace, means for passing a combustion gas through the gas space, a plurality of fluid heating tubes exposed to said combustion gas, and wherein at least some of the tubes have end portions thereof terminating outside of said gas space, the structure including a plurality of openings, each of the openings being sized to slidably receive a respective one of said tube portions while preventing movement thereof transverse to the central axis of the opening, header means flow connected to said end portions, wall means cooperating with the structure to form at least one compartment enclosing said header means, conduit means for conveying a portion of the combustion gas to said compartment, and means for maintaining the combustion gas within said compartment at a higher pressure level than that of said gas space thereby sealing the compartment from said gas space.
 2. The combination according to claim 1 wherein said last named means includes a duct situated outside of the gas space and communicating with said furnace and passageway, a fan disposed in said duct for recirculating a portion of the combustion gases from the passageway to the furnace, and said conduit means being flow connected to the duct at a point downstream gas flow-wise of said fan.
 3. The combination according to claim 1 wherein said conduit means is flow connected to the passageway. 